This invention relates to improvements in carbon/carbon friction material performance and chemical vapor infiltration (CVI) densification rate through the incorporation of ceramic additives. Specifically, colloidal ceramic additives, e.g. silica and aluminum oxide, were infiltrated into PAN and pitch-based carbon fiber preforms to achieve improvements in densification time or rate and performance characteristics.
Carbon/carbon brake discs have gained wide acceptance on both commercial and military aircraft. This trend can be attributed to the increased brake performance requirements for newer aircraft and the unique physical, thermal, and chemical properties of carbon. On the commercial side, the advent of wide-bodied jets required the need for improved brake materials simply because traditional steel brake systems were inadequate to absorb all the thermal energy created during landing. Steel could only be used if additional heat sink were available. But due to the demand for lower weight systems, this was not feasible. Also, carbon possesses very unique properties. Its heat capacity, thermal conductivity, high temperature strength and density make it the ideal material for the demanding conditions which often occur during aircraft landings. On the military side, weight and thermal properties also made carbon the material of choice. Only carbon's unique properties meet the performance requirements demanded for rigorous military applications.
General methods for the production of carbon/carbon composite materials, including brake friction materials, have been extensively described in both patents and the open literature e.g. Buckley, J. and Edie, D., eds., Carbon-Carbon Materials and Composites, Noyes Publications, Park Ridge, N.J., 1993. To summarize, two commonly used production methods exist. The first method comprises molding a carbon fiber composite with some carbonizable resin, typically phenolic; carbonizing the composite "preform" and then densifying the now porous material using CVI and/or resin impregnation processes. CARBENIX.RTM. 2000 series materials (AlliedSignal Inc.) are typical of this type. The second method comprises using textile methods to build up an all-fiber preform with subsequent densification using CVI. CARBENIX.RTM. 4000 series materials (AlliedSignal Inc.) are typical of this type.
In both types of brake discs, the final density is usually substantially less the theoretical density of graphitic carbon (2.2 g/cc) This can be attributed in small part to the different structural types of carbon comprising the brake disc (graphitic, glassy, and pyrolytic). Mainly this is due to the continued presence of interconnected pore networks and closed pores remaining because of incomplete densification. Further densification is possible if continuous pore channels are open to the surface of the brake disc. Low viscosity solutions such as furfuryl alcohol are sometimes used effectively to provide small enhancements in final density. But small enhancements don't come without increased processing and materials cost, consequently techniques such as these aren't widely used in industry. Up to 10-15 volume percent porosity does remain in the final brake disc microstructure. It is highly desirable to improve densification time or rate.
Carbon/carbon brake disc friction performance is dictated by carbon microstructure produced through processing. As discussed above, numerous types, or polymorphs, of carbon can be found in the final brake microstructure. The structure of pyrolytic carbon can be varied depending on the deposition parameters during CVI processing. Terms such as rough and smooth laminar have been used to describe certain types of CVI deposited carbon structures, and amounts are optimized for friction and wear applications. Physical properties such as tensile strength and hardness are important criteria when optimizing overall brake disc friction and wear. Additionally, heat treatment temperatures play a large role in determining final microstructure and hence performance. The amount of graphitization, for instance, can immensely effect frictional properties. Graphite can be a good solid lubricant if basal planes are oriented in the proper direction, which would have major effects on friction and wear properties. Overall brake performance will be controlled by the individual components; fibers and type of matrix material in contact with the friction surface. This poses an infinite number of ways in which performance specific material properties can be engineered into brake microstructures. It is highly desirable not only to improve densification rate but to improve the friction performance of the brake disc.